A TMR sensor otherwise known as a magnetic tunneling junction (MTJ) is a key component (memory element) in magnetic devices such as Magnetic Random Access Memory (MRAM) and a magnetic read head. A TMR sensor typically has a stack of layers with a configuration in which two ferromagnetic layers are separated by a thin non-magnetic insulator layer. The sensor stack in a so-called bottom spin valve configuration is generally comprised of a seed (buffer) layer, anti-ferromagnetic (AFM) layer, pinned layer, tunnel barrier layer, free layer, and capping layer that are sequentially formed on a substrate. The free layer serves as a sensing layer that responds to external fields (media field) while the pinned layer is relatively fixed and functions as a reference layer. The electrical resistance through the tunnel barrier layer (insulator layer) varies with the relative orientation of the free layer moment compared with the reference layer moment and thereby converts magnetic signals into electrical signals. In a magnetic read head, the TMR sensor is formed between a bottom shield and a top shield. When a sense current is passed from the top shield to the bottom shield (or top conductor to bottom conductor in a MRAM device) in a direction perpendicular to the planes of the TMR layers (CPP designation), a lower resistance is detected when the magnetization directions of the free and reference layers are in a parallel state (“1” memory state) and a higher resistance is noted when they are in an anti-parallel state or “0” memory state. Alternatively, a TMR sensor may be configured as a current in plane (CIP) structure which indicates the direction of the sense current.
A giant magnetoresistive (GMR) head is another type of memory device. In this design, the insulator layer between the pinned layer and free layer in the TMR stack is replaced by a non-magnetic conductive layer such as copper. A GMR-CPP configuration is more desirable than a GMR-CIP design for ultra-high density applications because a stronger output signal is achieved as the sensor size decreases, and the magnetoresistive (MR) ratio is higher for a CPP structure.
In the TMR stack, the pinned layer may have a synthetic anti-ferromagnetic (SyAF) configuration in which an outer pinned layer is magnetically coupled through a coupling layer to an inner pinned (AP1) layer that contacts the tunnel barrier. The outer pinned (AP2) layer has a magnetic moment that is fixed in a certain direction by exchange coupling with the adjacent AFM layer which is magnetized in the same direction. The tunnel barrier layer is so thin that a current through it can be established by quantum mechanical tunneling of conduction electrons.
A TMR sensor is currently the most promising candidate for replacing a GMR sensor in upcoming generations of magnetic recording heads. An advanced TMR sensor may have a cross-sectional area of about 0.1 microns×0.1 microns at the air bearing surface (ABS) plane of the read head. The advantage of a TMR sensor is that a substantially higher MR ratio can be realized than for a GMR sensor. In addition to a high MR ratio, a high performance TMR sensor requires a low areal resistance RA (area×resistance) value, a free layer with low magnetostriction (λ) and low coercivity (Hc), a strong pinned layer, and low interlayer coupling (Hin) through the barrier layer. The MR ratio (also referred to as TMR ratio) is dR/R where R is the minimum resistance of the TMR sensor and dR is the change in resistance observed by changing the magnetic state of the free layer. A higher dR/R improves the readout speed. For high recording density or high frequency applications, RA must be reduced to about 1 to 3 ohm-um2.
A MgO based MTJ is a very promising candidate for high frequency recording applications because its tunneling magnetoresistive (TMR) ratio is significantly higher than for MTJs with an AlOx or TiOx tunnel barrier. An MR ratio of ˜200% has been achieved at room temperature in epitaxial Fe(001)/MgO(001)/Fe(001) and polycrystalline FeCo(001)/MgO(001)/(Fe70CO30)80B20 MTJs as reported by Yuasa et. al in “Giant room-temperature magnetoresistance in single crystal Fe/MgO/Fe magnetic tunnel junctions”, Nature Materials 3, 868-871 (2004) and by Parkin et al in “Giant tunneling magnetoresistance at room temperature with MgO (100) tunnel barriers”, Nature Materials 3, 862-867 (2004). S. Yuasa et al. also reported in “Giant magnetoresistance up to 410% at room temperature in fully epitaxial Co/MgO/Co magnetic tunnel junctions with bcc Co(001) electrodes”, Applied Phys. Letters 89, 042505 (2006) that a TMR ratio as high as 410% at RT can be achieved in a fully epitaxial Fe(001)/Co(001)/MgO(001)/Co structure. In addition, Djayaprawira et. al showed that MTJs having a CoFeB/MgO(001)/CoFeB structure made by conventional sputtering can also have a very high MR ratio (230%) with the advantages of better flexibility and uniformity in “230% room temperature magnetoresistance in CoFeB/MgO/CoFeB magnetic tunnel junctions”, Physics Letters 86, 092502 (2005). However, RA values in the MTJs mentioned above are in the range of 240 to 10000 ohm-um2 which is too high for read head applications.
K. Tsunekawa et. al in “Giant tunneling magnetoresistance effect in low resistance CoFeB/MgO(001)/CoFeB magnetic tunnel junctions for read head applications”, Applied Physics Letters 87, 072503 (2005) found a reduction in RA by inserting a DC-sputtered metallic Mg layer between a bottom CoFeB layer and RF-sputtered MgO. The Mg layer improves the crystal orientation of the MgO(001) layer when the MgO(001) layer is thin. The MR ratio of CoFeB/Mg/MgO/CoFeB MTJs can reach 138% at RA=2.4 ohm-μm2. The idea of metallic Mg insertion was initially disclosed by Linn in U.S. Pat. No. 6,841,395 to prevent oxidation of the bottom electrode (CoFe) in a CoFe/MgO/NiFe MTJ structure where the MgO tunnel barrier is formed by RF sputtering. Furthermore, a Ta getter pre-sputtering prior to the RF-sputtered MgO layer can achieve a 55% TMR ratio with RA=0.4 ohm-μm2 as reported by Y. Nagamine et al. in “Ultralow resistance-area product of 0.4 ohm-μm2 and high magnetoresistance above 50% in CoFeB/MgO/CoFeB magnetic junctions”, Applied Phys. Letters 89, 162507 (2006).
Although a high MR ratio and low RA has been demonstrated in MTJs based on a MgO tunnel barrier, the remaining challenge is to make the free layer magnetically soft (low Hc and low λ) without sacrificing TMR ratio to meet the requirement of high performance read head applications. In order to achieve a smaller Hc but still maintain a high TMR ratio, the industry tends to use CoFeB as the free layer in a TMR sensor. Unfortunately, the magnetostriction (λ) of a CoFeB free layer is considerably greater than the maximum acceptable value of about 5×106 for high density recording applications. A free layer made of a CoFe/NiFe composite has been employed instead of CoFeB because of its low λ and soft magnetic properties as described in US Patent Application 2007/0015293. In related US Patent Application 2007/0111332, a Mg/MgO/Mg barrier layer, CoFe/NiFe free layer, and a low temperature anneal of 250° C. to 300° C. is disclosed. However, when using a CoFe/NiFe free layer, the TMR ratio will degrade.
U.S. Patent Application 2007/0063237 describes a first annealing to set the pinned layer and a second annealing at a lower temperature and lower magnetic field to achieve an offset angle for the free layer magnetization with respect to the pinned layer magnetization direction.
U.S. Patent application 2006/0255383 discusses changes in MTJ resistance and TMR ratio as a function of annealing temperature between RT and 350° C.
In U.S. Patent Application 2007/0012952, an annealing process involving a temperature between 150° C. and 250° C. and a magnetic field from 200 to 2000 Oe is disclosed. U.S. Patent Applications 2007/0263429 and 2007/0176251 teach a higher temperature annealing in the range of 340° C. to 360° C.
It is also known in the art that a CoFe/CoFeB, CoFeB, or CoB free layer is able to achieve high dR/R and low λ with relatively low annealing temperature (<300° C.). However, the magnetic properties of a CoFeB. CoFe/CoFeB, or CoB free layer are very sensitive to the annealing process due to structural change of the amorphous CoFeB and CoB materials during anneal. With high annealing temperature and long annealing time, a high dR/R can be realized because of crystallization of the amorphous CoFeB or CoB layer, especially near the interface with the tunnel barrier. Unfortunately, crystallization of CoFeB or CoB may lead to harder magnetic properties such as high Hc in Co-rich CoFeB or CoB. One must also be concerned about the effect of the annealing temperature on pinning strength in MTJ devices. Therefore, the annealing process for MTJs based on MgO tunnel barriers and amorphous free layers needs further improvement in order to meet all the requirements of a high performance read head.